Too big of a stepper

[Perp]

In building (planning) a plasma table, where is the line at which a stepper is too big. Not considering cost, only performance. I have read a lot about gearing down for torque and not having enough acceleration to make good details, so to me using a 1200oz stepper would be a great idea. but what are the issues with going overkill on the stepper.Is it juts the added weight? Is it harder on the power source/drivers? Maybe it breaks pinion teeth? or is it just the cost that keeps most from using it?

Also I have gotten the impresion that if you have athc, then the accuaracy of the table (flatness) is much less important. is this correct?

Thanks

Doug

[Torchhead]

The major problem with big steppers is getting them to not have serious resonance points with the drives. Those motors have such low impedance that they are a problem to run. The Gecko drives have a special jumper for bigger (42 frame motors) but even with that there have been reports of lost steps.

Athc has nothing to do with the accuracy (or lack of) of a table. All the ATCH does is maintain a constant cut gap as you move. Yes, it will compensate for un-level cutting grids and warped material so you don't need to get out your level and shim stock up to get a good cut.

The accuracy of a cut is determined by a lot of things and plasma is not a "precision" cutting system. You can hold .020 on a good day with a 30,000 deg flame vaporizing metal. The places where accuracy helps is cutting curves and circles. A loose table or one with course steps will cut a ragged curve. If you want to do quality work then a table that has a base accuracy of .005 to .010 is a part of that. Don't confuse accuracy with resolution. While your accuracy can never be better than your resolution (defined as the smallest distanct you move in one step) it most often is a lot worse because of mechanics, backlash, etc)

I think a 1200 oz stepper is fine for a full sized mill conversion. For most router tables a 640 oz stepper is quite sufficient (depending on the gearing) and for plasma tables with light gantries an no radial cutting forces even smaller motors work fine.

It's like having a 1200 HP motor in a Hugo. It may be impressive but hardly practical.

[Terry G]

Torchhead,

When you suggest a specific torque for a certain table, (640 oz for a router table), would that be the final torque the table would see if using gear reduction with servo motors?

I have never used stepper motors before, so I am not sure if you would even use gear reduciton with steppers, or have them drive the table direct.

Thanks.

[Perp]

Thanks for the input Torchhead. When I said accuracy, i should have been more clear. I was referring only to flatness the rails. It sounds like if one rail has a hump in it of say 0.02" and it is gradual the ATHC will compensate for this just as it would for a high slat. is this correct? I do realize if the rails are not parallel or if they are not straight (side to side) then you have no chance at any accuracy.

Like Terry asked, when you state 640oz you are referring to motor torque (stall torque I believe??) and not final reduction output, correct. Thanks for your help

Also, for the THC you sell, if I were to purchase this what else is required to make it functional, as far as electrical and mechanical?

[Torchhead]

motor torque for a stepper....more of a size that anything. With rack and pinion the motor needs to be geared or belt reduced 3:1 to get back to the original motor torque because of the ratio in the pinion. Even fairly small diameter pinions are PI times the diameter. If you know the pitch of your rack and the pitch of the pinion (Teeth per inch) then the you can calculate the distance the pinion will move the load in one rev. Take a pinion with a 1" DP. It would move about 3.14 inches linearly in one rev. Hook up a stepper direct and spin it at 600 RPM and you have 1800 IPM speeds. Impressive but not very usable. Those kind of speeds throwing a mass about (even a moderate one) will be hard on anything lightly built. Build it heavy and the forces get worse.

Unlike a router, where depth into the cut is usually more critical, a good THC will compensate for vertical deviations. It can easly compensate for a rise of 1" over 6". You need to make sure the design takes into account the piercing process where the pierce needs to be made at a precise height above the material each time and for a variable delay based on feedback from the plasma rather than a preset pierce delay.

[Perp]

That is very informative and I now understand the reduction. Thank you. I assume that if it were direct drive your resolution would be one third of what you would expect if it were reduced (assuming3:1 reduction), Correct?

Also about the THC, what does it need to be complete? a small stepper and linear track and some wiring or is it more. also, do you recommend a Z axis in addition to the THC, I think i read this somewhere else as it is needed to set the initial height to accommodate different thickness materials. Sorry for all the basic questions but i want to have a solid foundation of understanding here.

Thanks again
Doug